This invention pertains generally to the field of high power diode laser arrays.
High power diode laser arrays are now in use for a variety of technological applications. Commercially available diode array systems with high output power (greater than 1 watt) are currently relatively expensive. Although the cost of such systems is likely to decrease over time, a typical current cost for a 15 watt diode array is in the range of $25,000. Clearly, the users of such arrays wish to have as much usable laser light as possible. Unfortunately, much of the light that is emitted is not usable for many applications.
One important use of high power diode lasers is for producing laser polarized noble gases for medical magnetic resonance imaging (MRI). As an example, a 15 watt laser array can be used to optically pump the noble gas sample. The user tunes the array to a wavelength at which the gas in the cell is activated, e.g., 795 nanometers where rubidium is used. Unfortunately, like all high power diode laser systems now available, the laser not only puts out laser light at the desired 795 nm, but also a spread of wavelengths around 795 nm. While the peak of the spectrum may be at 795 nm, the 15 watts of output power is typically spread over several nanometers, typically displaying a Gaussian-type curve of laser power versus wavelength centered at the desired wavelength. Thus, only a fraction of the 15 watts of output power is usable by the cell. Under reasonably attainable conditions, generally only the light which is between about 794.9 and 795.1 nanometers is useful. The vast majority of the output power of the laser is outside this range and is wasted; typically of the 15 watts of power produced by the laser system, only 1 or 2 watts may fall within the useful range.
In accordance with the present invention, a high power diode laser array system utilizes an external cavity to narrow the spectral width of a high power diode laser array to change the output power normally produced by the laser array from a broad spectrum to a very narrow spectrum, so that more power is concentrated over a narrow spectral range which falls within the usable range for a particular application, such as optical pumping of noble gas samples for MRI. The total power output of the laser system is reduced a moderate amount from the output power provided from the laser array alone, but is concentrated within a narrow spectral range to provide a much higher equivalent power laser system.
The high power diode laser array system in accordance with the present invention includes a high power diode laser array comprising multiple laser diode emitters arranged in approximately a straight line. A cylindrical collimating element is positioned to receive the output of the laser diode array and provide an output beam that is collimated in a direction perpendicular to a direction along the length of the array of emitters. A diffraction grating is mounted to receive the collimated beam from the collimating element on a beam path. The diffraction grating may be arranged in a Littrow cavity configuration, oriented at an angle to the incident beam such that a portion of the light in the beam incident on the grating is directed back on the beam path to the collimating element and is focused on the array of laser diode emitters to provide feedback thereto to narrow the spectral range of the laser light output. A portion of the beam may be directed by the diffraction grating to provide a useable output light beam from the laser system or otherwise directed out of the cavity. The diffraction grating may also be arranged in a Littman-Metcalf cavity configuration; with the light directed from the grating to a mirror and back to the grating to form the light fed back to the array. A magnifying system formed of optical elements receives the beam from the cylindrical collimating element and images the emitters of the array in the long dimension of the array that is along the length of the array onto the diffraction grating or the mirror in a Littman-Metcalf configuration, with a selected magnification factor, the grating having grooves which are aligned parallel to the long dimension of the array of emitters. The cylindrical collimating element may comprise, for example, a cylindrical lens. The magnifying system is preferably formed of lenses arranged as a telescope with a selected magnification factor. A polarization rotation element may be mounted in the beam path from the collimating element to the diffraction grating. The polarization rotation element is oriented such that the light on the beam path passed therethrough to the diffraction grating is oriented with respect to the diffraction grating to provide a selected efficiency of the diffraction grating and to select the amount of light directed back by the diffraction grating toward the laser diode array to provide effective feedback without damaging the laser diode array. The polarization rotation element (e.g., a half wave plate) can be mounted for rotation about an axis parallel to the output beam from the collimating element to allow rotation of the polarization rotation element to select the amount of feedback to the laser diode.